For a conventional diesel direct injection combustion system system there are multiple unique combinations of combustion system parameters that can provide reasonably well optimized combustion. These parameters include the number of spray holes, injection duration, injection pressure level, spray angle, hole diameter and spray hole taper or inlet rounding, swirl level, chamber diameter and general combustion chamber shape. The selection made is a factor of many variables including the manufactures experience and general design philosophy. There are also trade-offs to be decided relative to engine load and speed characteristic, maximum engine speed, torque rise requirements, exhaust emission strategy, cost, hardware constraints…
Historically manufacturers developed unique hardware combinations they believed gave them competitive advantage and reflected the strategy in the combustion systems of multiple engine designs over many years. Examples of this are Detroit Diesel with large open shallow bowl, swirl supported combustion with high pressure unit injectors, Cummins with zero swirl engines having high injection pressure (P-T system) and large diameter combustion bowls, and MAN with spherical bowl pistons, very high swirl and single hole nozzles and low injection pressure. Good combustion systems have been built with durations of injection between about 20 and 40 degrees suiting the constraints of the designs and Swirl Ratios from zero to over 12 (opposed piston).
Through history, successful combustion system trends have evolved much as with natural selection in nature. Systems are benchmarked, investigated, and developed with the most competitive systems maintaining a presence in the market. In recent history the shift towards common rail injection at increasingly higher pressures with increasing numbers of spray orifices and reduced swirls levels seems to be trending. One partial exception is a Volvo heavy duty combustion system with a “WAVE” Piston. Using very low induced swirl this system with a large diameter bowl but with only a 6 hole nozzle! The unique feature of the piston are “bumps” around the periphery of the bowl.
At first the use of a 6 hole nozzle appear to defy the trend of increased nozzle holes number with decreased swirl to achieve a good combustion match (see Swirl) . However with more thought, this system is very similar to the Cummin’s “no swirl” combustion systems. The Cummins system typically used 7 hole nozzles and zero swirl. This system used a large diameter piston bowl and high injection pressure and small nozzle holes for good spray break-up and high air entrainment. When the sprays reaches the bowl they are first deflected tangentially and then as two adjacent plumes meet are deflected back towards the center of the chamber. This is highly driven by the fact that a spray plume is moving mixture of fuel and air and there must be corresponding inward flows within the chamber as the spray moves outward. The Volvo system appears to provide an improvement by providing bumps to assist in the inward redirection of the spray and the avoidance of spray to spray mixing interference that might cause regions of local over richness.
GM’s “Polyfoil” combustion systems, Ref. US Patents 9,267,422 and 9,091,199 provide a similar approach to using piston shape to harness the momentum of the spray to create organized air-fuel mixing. See figure below.
Another combustion system I find interesting is that of the Gardner 6LXB and 6LXC truck engines. In there day as naturally aspirated engines they had not only extremely good fuel consumption, but achieved very high BMEPs . The engine utilized a hemispherical bowl and a 2 valve cylinder head. The system used a pump-line – nozzle injection system of I assume relatively low pressure. My research indicates use of a 3 hole nozzle with a 150 degree included angle of the sprays.
To achieve such good fuel consumption there are probably multiple factors involved. No doubt the favorable stroke to bore ratio and low friction and parasitics are factors. With low injection pressure, relative to high pressure systems like Cummins and Detroit Diesel, fuel system pumping work would be low and energy to vaporize the fuel would come primarily from heat transfer from the piston. This also would provide some degree of piston cooling. With the relatively low compression ratio, cylinder pressures would also the lower thus minimizing fiction. Longer ignition delay may have also added in soot reduction from increased pre-mixing.
While I have no data or direct experience with the engine, I believe the Gardner engine may have had high exhaust HC emissions and perhaps cold smoke. A general observation I have made over my career is that what I refer to as wall mixing systems have higher HC emissions than systems with primarily highly atomized airborne sprays. This is particularly true at light loads and immediately after cold start. A good question would be if advanced fuel systems that could transition between airborne and wall impinged fuel sprays could at least partially solve the problem. Pintaux nozzles were a partial attempt to do this for the MAN system. Use of steel pistons with hotter surface temperatures or use of ceramic coatings could also partially offset the issue. Use of HC storage systems, oxidation catalysts, and fast warm-up systems with hot internal exhaust gas recirculation (EGR) would also alleviate the HC problem.
Despite having spent much of my career in the exploitation of higher injection pressures to advanced combustion systems, I find the possibility of an advanced “wall mixing” systems an interesting possibility – perhaps as part of a solution for lower cost applications.